The catch is that nobody is entirely certain it exists. Local tradition holds that the passage was closed up at some point, and today there is nothing visible at ground level to confirm or contradict the story. It is, in a sense, an archaeological site defined almost entirely by its absence.

A rath is a roughly circular enclosure, typically bounded by an earthen bank and ditch, used as a farmstead during the early medieval period in Ireland. Souterrains, the underground stone-lined passages sometimes found within them, were built for a variety of purposes, most likely storage and refuge. They are dry, cool, and, when sealed, effectively invisible. The one at Shannera fits that last quality rather well. Whether it was deliberately blocked by those who built it, or closed up at some later point for reasons now forgotten, is not recorded. What survives is only the local memory that something is there, underground, waiting.